. Zoology : for students and general readers . Zoology. CBLLS AND TISSUES. 7 ferentiation of organs involves the division of physiological labor. Tissues.—Of the different kinds of tissues there is, first, epithelialtissae (Fig. 4), consisting of cells of the most prim- itive form, and placed side by side, forming a layer. All the organs develop originally from ejntheUum, which is the prim- itive cell-structure and forms the tissues of the germ-layers. Epithelial cells form the skin of animals, and also the lining of the digestive canal. The cells of the latter may, as in sponges, bear a gener
. Zoology : for students and general readers . Zoology. CBLLS AND TISSUES. 7 ferentiation of organs involves the division of physiological labor. Tissues.—Of the different kinds of tissues there is, first, epithelialtissae (Fig. 4), consisting of cells of the most prim- itive form, and placed side by side, forming a layer. All the organs develop originally from ejntheUum, which is the prim- itive cell-structure and forms the tissues of the germ-layers. Epithelial cells form the skin of animals, and also the lining of the digestive canal. The cells of the latter may, as in sponges, bear a general resemblance to a flagellate infuso-. Fig. 4.—Vertical section through the Bkiu of an embryonic sharlf. showing at ^the epithelial cells, forming pavement epithelium ; and at e, columnar epithelium.—After Gegenbaur. rian, as Codosiga, or they may each bear many hairs, called cilia, which by their constant motion maintain currents of the fluids passing over the surface of the epithelium. The tissue forming glands is simply modified epithelium. C'oHnective tissue is formed by isolated rounded or elon- gated cells with wide spaces between them filled with a ge- latinous fluid or protoplasm, and occurs between muscles, etc. This tissue forms in the vertebrates the " notochord " — , a rod supporting the bodies of vertebrate embryos. Gelatinous tissue is a vai-iety of connsctive tissue found in the umbrella of jelly-fishes {Aurelia, etc.). Fibrous and elastic tissue are also varieties of connective tissue. Cartilaginous tissue is characterized by cells situated in a. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Packard, A. S. (Alpheus Spring), 1839-1905. New York : Henry Holt
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectzoology, bookyear1879
